The inventors herein and others developed a module of filtering hollow fibre membranes which they described in U.S. Pat. No. 5,248,424, issued on Sep. 28, 1993. In this module, hollow fibre membranes are held in fluid communication with a pair of horizontally spaced headers to form modules in a variety of configurations in which the fibres vary from being substantially horizontal to substantially vertical. To produce permeate, transmembrane pressure ("TMP") is provided by suction on the lumens of the fibres.
Subsequently, further shell-less membrane modules based in part on similar principles appeared with hollow fibre membranes in both substantially vertical and substantially horizontal orientations. Shell-less modules with membranes oriented vertically are shown in U.S. Pat. No. 5,639,373 issued to Zenon Environmental Inc. on Jun. 17, 1997; U.S. Pat. No. 5,783,083 issued to Zenon Environmental Inc. on Jul. 21, 1998 and PCT Publication No. WO 98/28066 filed on Dec. 18, 1997 by Memtec America Corporation. In these modules, the horizontally spaced headers are replaced by headers spaced vertically only.
Shell-less modules with membranes oriented horizontally are described, for example, in U.S. Pat. No. 5,480,553 issued to Mitsubishi Rayon Co., Ltd on Jan. 2, 1996; European published application EP 0,931,582 filed on Aug. 8, 1997 by Mitsubishi Rayon Co., Ltd and in an article, "Development of a tank-submerged type membrane filtration system", by K. Suda et. al. of Ebara Corporation published in Desalination 119 (1998) 151-158.
Despite these developments, membrane filtration technology is not widely used for creating potable water. Sand filters are still used more often, largely because of their lower cost for a given capacity. For example, tests by the Ebara Corporation were reported in the article mentioned above. While the authors achieved stable operation over extended periods of time, the tank superficial velocity (the flux of permeate, typically in m.sup.3 /h, divided by the tank footprint, typically in m.sup.2) was only about 1.7 m/h. In comparison, a typical sand filtration system has a tank superficial velocity of 5-10 m/h allowing for the use of much smaller tanks, a significant cost in a large municipal or industrial system. Modules of vertical membranes produced by Zenon Environmental Inc. have been operated to produce tank superficial velocities of over 10 m/h but while using strong membranes supported with a substrate and fairly intense aeration. Both the complex membranes and the intense aeration increase the cost of such technology. Finally, the cost of maintaining known membrane modules is also a concern. In particular, leaks or defects in a single membrane are difficult to locate or isolate and often require large sections of a filtration system to be taken off line for their repair.